1. Field of the Invention
The present invention relates to an improved surface acoustic wave device which comprises LiTaO.sub.3 and diamond.
2. Description of the Related Art
A surface acoustic wave device (SAW device) is a device which utilizes the acoustic wave propagation and the piezoelectricity on the surface of particular solid materials. The SAW device has excellent temperature stability, durability, and phase characteristics. Thus, there are certain demands, in the field of the advanced communication technology, for SAW devices which can be used in high frequency bands of more than 3 GHz, such as band-stop filters, resonators, delay devices, signal processing devices, convolvers, and functional elements for optoelectronic devices. For example, the band-stop filter with wider bandwidth is necessary for the equipments for cellular communications which are typically operated in high frequency bands of more than 3 GHz.
The SAW devices typically comprises interdigital transducers(IDT) for generating and detecting the surface acoustic wave. The operation frequency(f) of a SAW device is given by the equation :f=V/.lambda., where V is the wave propagation velocity in the SAW device, and .lambda. is the wavelength of the surface acoustic wave. The operation frequency of more than 3 GHz is required for SAW device to be used for the band-stop filters with wider bandwidth. The wavelength .lambda. is generally proportional to the width (d) of electrodes of the interdigital transducer.
Because of difficulties on microfabrication technique, the electrode with the width (d) of less than 0.5 .mu.m is difficult to be obtained, thus it is difficult to achieve desirable operation frequency (f) of more than 3 GHz by decreasing wavelength .lambda.. Therefore, the SAW device with higher propagation velocity V of not less than 7,000 m/s is required for being applied to the operation at the frequency of 3 GHz or higher.
The energy transform (piezoelectrical) efficiency is also important for SAW device to be used for the advanced communication equipments. The effective coupling coefficient(K.sup.2) is an index to conversion efficiency of the converting of electrical energy into mechanical energy on the surface of the device, and higher K.sup.2 of at least 2% or more is thus required for the use of the SAW device in particular in the cellular communication equipments with less energy loss.
The theoretical and the experimental evaluation on surface acoustic wave properties of SAW devices, which includes ZnO for piezoelectrical material and saphire, is disclosed by Mitsuyu et.al.(Mitsuyu,T., S Ono and K Wasa, J. Appl. Phys, 2464-2470,51(5), May 1980). The reference teaches that the saphire-ZnO SAW device can be used for filter with 1050 MHz center frequency. However, Mitsuyu did not teach the SAW device which can be operated at the frequency of 3 MHz or higher. Consequently, a different constitution of SAW device is necessary for the operation at higher frequency.
Diamond is a material which have higher sonic propagation properties than other materials for SAW device, such as saphire.
LiTaO.sub.3 has advantages that the chemical resistance is superior to that of other piezoelectrical materials such as ZnO. Therefore, the SAW devices comprising LiTaO.sub.3 layer as piezoelectrical material and diamond are expected in particular to be used for cellular communications, because of its durability. An SAW device having a laminate structure of a diamond layer and an LiTaO.sub.3 layer is disclosed in the art (Japanese Laid-open Patent Application No.62911/1989). The references, however, did not teach the SAW device which can be used for the communication equipments of higher frequency bands, such as cellular phones.
An objective of the present invention is therefore to provide a SAW device comprising an LiTaO.sub.3 layer as a piezoelectric material, which have improved propagation velocity (V), and improved effective coupling coefficient (K.sup.2).
Another objective of the present invention is to provide a SAW devices which has excellent properties for the operation at the frequency of 3 MHz or higher.